The invention relates to a method for nondestructive testing of pipes.
Nondestructive methods for testing metallic pipes for surface flaws, such as eddy current tests or ultrasound tests, are known for quite some time and have proven to be successful.
The ultrasound test is applied, for example, to monitor particularly the compliance of the pipe with the required wall thickness during production and to detect any discontinuities present in the pipe wall, such as laminations, cracks, notches, scrap marks or other surface flaws.
During the test, ultrasound pulses are excited in the wall according to the pulse-echo method starting at the exterior surface of the pipe, with the signals reflected at the interior surface of the pipe being received.
The thickness of the pipe wall can be calculated from the transit time of the signal and the sound velocity of the material to be tested. This method is typically used during production and automated for both magnetizable and non-magnetizable pipe materials.
This is done by accurately scanning the perimeter of the pipe over the entire pipe length. In addition to determining the wall thickness (WD) and the outside diameter (Da), the interior and exterior surface of the pipe is also checked for flaws.
The known leakage flux test can also be used as an additional or alternative method for detecting surface flaws. The constant-field leakage flux test, which is predominantly employed for the detecting exterior flaws on the pipe wall, is used for pipes made of ferromagnetic steel to detect longitudinal discontinuities near the surface, such as cracks, scales, or bulges.
With these known nondestructive testing methods, the wall thickness and diameter of the pipe can be accurately measured and the perimeter of the pipe can be checked for flaws along the entire pipe length with a high resolution of several centimeters.
During the evaluation of the measurement values it is checked that a certain predetermined flaw threshold of x % of the nominal wall thickness (reference flaw depth, RFT) is not exceeded.
In a continuous manufacturing process used to produce, for example, seamless pipes, the flaws identified during the flaw test and marked on the pipe must be repaired, which is typically done during production by eradicating the flaw through grinding or milling.
However, certain conditions must be satisfied, because the wall thickness must not be less than a certain minimum wall thickness.
Hitherto, the identified flaws were initially ground either on the outside or on the inside of the affected pipe, whereafter the pipe was again moved through the test setup to verify, on one hand, that the pipe is now free of flaws and, on the other hand, that the required minimum wall thickness is maintained in the reworked flaw region, so that the pipe can be released.
With this approach, however, it can disadvantageously be ascertained only after mechanical machining and after subsequently repeating the test if the pipe is indeed free of flaws and has the required minimum wall thickness at the reworked location, or if the pipe needs to be cut or in extreme situations even scrapped.
This disadvantageous situation is due to the problem that in a continuous manufacturing process, the residual wall thickness can manually either not be determined at all or only with great difficulty.
For example, in a discontinuous production process, where the operating speed is less important, the wall thickness can be measured manually using ultrasound (US) at the location of a flaw to determine if reworking is feasible when taking into account the minimum wall thickness to be maintained.
This is, on one hand, quite time-consuming, which is unacceptable in a modern continuously operating production line and, on the other hand, requires additional personnel trained to perform these measurements.